Polyurethane (PUR) products frequently occur in industry, in particular in manufacturing and handling polyurethane foam, elastomers, adhesives and lacquers. Polyurethane is produced by the reaction of a bifunctional isocyanate with a polyfunctional alcohol. The satisfactory technical qualities of polyurethane have resulted in a large increase of its use and application fields during the last decade. In connection with thermal decomposition of polyurethanes, however, the formation of isocyanates, aminoisocyanates and amines might occur, and extremely high contents can be found in air, e.g. when welding automobile sheet steel. Besides the known types of isocyanate, also new types of aliphatic isocyanates have been detected, in connection with e.g. heat treatment of car paint. Most of the isocyanates formed have been found to be represented by so-called low-molecular isocyanates. During short periods of time (peak exposure) particularly high isocyanate contents can be present, as is the case, for instance, when welding. Of all the dangerous substances on the limit value list, isocyanates have the lowest permissible contents. Exposure to this new type of isocyanates was previously unheard of. Isocyanates in both gas and particle phase have been detected in connection with welding, grinding and cutting of painted automobile sheet steel, and respirable particles in high contents containing isocyanates have been detected. In thermal decomposition products of painted automobile sheet steel, detection has been made of, among other things, methyl isocyanate (MIC), ethyl isocyanate (EIC), propyl isocyanate (PIC), phenyl isocyanate (PhI), 1,6-hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 2,4- and 2,6-diisocyanate toluene (TDI) and 4,4-methylene diphenyldiisocyanate (MDI).
In thermal decomposition of phenol/formaldehyde/urea-(FFU)-plastic, isocyanic acid and methyl isocyanate are formed. FFU plastic is used, among other things, in wood glue and as a binder in mineral wool (and bakelite), which is frequently used as insulation for ovens and furnaces in industrial and domestic use. New fields of application in which exposure to isocyanates has been detected are the soldering and processing of printed circuit boards in the electronic industry, the welding, grinding and cutting of painted sheet steel in the automobile industry and the welding of lacquered copper pipes. Isocyanates have a varying degree of toxicity to the organism depending on their chemical and physical form. As a result, the hygienic limit values have been set at an extremely low level in all countries. For the exposed individual, the degree of exposure to isocyanates varies considerably in different operations during a working day and in connection with breakdowns. Thermal decomposition products from PUR constitute a special problem, since new and completely unknown isocyanates are formed, whose toxicity has not yet been analyzed in a satisfactory manner. Furthermore, the increasingly sophisticated measuring methods have revealed exposure to isocyanates in an increasing number of operations in industry.
To sum up, there is a number of operations in numerous working areas where people are daily exposed to or at risk being exposed to isocyanates at a varying degree. Considering the ominous tendency of isocyanates to cause respiratory diseases and the fact that there are some carcinogenic substances among the thermal decomposition products of polyurethane, e.g. 2,4-diamine toluene (TDA), 4,4-methylene diamine (MDA) and MOCA, it is very important to measure in a reliable, sensitive and rapid manner any presence of isocyanates, but also other decomposition products dangerous to health, in environments where there is such a risk.
Due to the high degree of reactivity of the isocyanates with other substances containing active hydrogen, the major part of the methods utilized for measuring in air flows are based on derivatisation in connection with the sampling step in order to protect the isocyanate group and allowing a selective determination of the isocyanates. A number of reagents and methods have been presented for the determination of isocyanates. However, there is only a limited amount of information about the reaction rate of isocyanates, and losses due to the presence of interfering substances has been reported, for instance, for 1-(2-methoxyphenyl)piperazine (2 MP) and MAMA as derivatisation reagents for 2,4- and 2,6-TDI. A method recently developed by the present inventor has a number of advantages in comparison with the above-mentioned MAMA method. This new method, which is called the DBA method due to the use of di-n-butylamine as reagent, allows the analysis of several new types of isocyanates and has been suggested as an international ISO reference method. The DBA method is based on the gathering of isocyanates in impinger bottles containing DBA in toluene and having a filter which is coupled in series and situated after the impinger bottle in the flow direction. In a sampling process, DBA solution and toluene are added to an impinger bottle. Subsequently, the sample flow is calibrated. An air flow is drawn through a tube immersed in the reagent solution, and isocyanates in the air flow react with DBA in the solution. Non-reacted gaseous isocyanates which have passed the solution are drawn through a filter which is provided with a reagent and arranged in connection with the suction device. Thus on this filter isocyanates which have not reacted with the reagent solution are bound. After completed sampling, the DBA solution with bound isocyanates is conveyed to and the filter is applied to one and the same test tube for further transport to an analysis step. Impinger bottles containing 10 ml 0.01 mole DBA in toluene have been used. Deuterium-labeled isocyanate DBA derivates are added to the samples and used as internal standards. Carbamate esters are formed by adding 2 ml 5 M NaOH, 10 μl pyridine and 50 μl ethyl chloroformate to the samples. The so-called DBA method has been tested for isocyanates in connection with spray painting with two typical biuret and isocyanurate adducts, HDI, IPD, polymeric MDI, TDI and thermal decomposition products from PUR plastic. High reaction rates for the reaction of the isocyanates with DBA have been observed, and the method is not sensitive to interfering substances. Since DBA is easy to eliminate in connection with the processing of the sample, the subsequent chromatographic determination is facilitated, which allows the use of the reagent in high contents. Before the chromatographic determination, the organic phase is separated and evaporated until it is dry. The rest is dissolved in 500 μl acetonitrile, after which the solution is injected into a liquid chromatographic (LC-mass-spectrometric (MS) system.
Other methods used for the determination of isocyanates have a number of drawbacks. Among other things, isocyanates which are present in both gas phase and particle phase in the air flow cannot be bound to the reagent in a satisfactory manner. Isocyanates which are present on and/or in particles, such as dust, will not be completely accessible to analysis, but will be polymerized to a kind of lump. Moreover, the reaction of the reagent with isocyanates is slow and negatively affected by interference from other substances present. In addition, the minimal sampling volume is about 0.5 l air, whereas the air flow which is obtained by means of a battery-operated air pump usually amounts to about 1 l/min. Furthermore, conventional sampling devices require manual adding of solvents and reagents as well as manual dismounting to convey the reagent liquid and the filter with bound isocyanates to the final analysis test tube. Another drawback is that such a sampling device can be tampered with to obtain false results.
In view of this, there is a great demand for an improved device and an improved method for sampling isocyanates, but also other products dangerous to health, such as aminoisocyanates, amines, isothiocyanates, anhydrides and carboxylic acids, in a rapid, reliable, precise and tamperproof manner.